1. Field of the Invention
The present invention relates to an optical pickup apparatus, and more particularly to an optical pickup apparatus for recording data on and/or reproducing data from recording mediums with different formats by using a single optical system.
2. Background of the Related Art
As a result of continuous efforts to increase a recording capacity of an optical disk, a digital video disk (hereinafter referred to as DVD) having a mass storage recording capacity in comparison to a compact disk (hereinafter referred to as CD) has been developed. Further, a high-density disk (hereinafter referred to as HD) for recording and/or reproducing data in high density (twice the density of an CD) by utilizing an existing CD and a drive therefor are currently under development. Compared with the CD, the DVD has not only a high recording density (i.e., track pitch) but also a shorter distance from the surface of the disk to the surface for recording data. Therefore, an optical pickup apparatus for a DVD has difficulty in recording or reproducing data from a CD. This is primarily because of spherical aberration (SA) which is generated when the distance between the surface of the optical disk and the surface for recording data changes, and coma aberration (CA) which is generated when the optical disk is tilted, and astigmatism due to defocusing.
Spherical aberration causes the intensity of a main lobe of an optical beam by a data recording medium area to be relatively greater than the intensity of a side lobe by an area other than the data recording medium, and as a result thereof, an interference phenomenon between data tracks is generated. Moreover, coma aberration and astigmatism make an optical system unstable, and deteriorates optic characteristics thereof. The spherical aberration, the coma aberration, and the astigmatism are dependent on the range of distances between the surface of the optical disk and the surface for recording data, refractivity, numerical aperture (NA) of an objective lens, amount of defocusing, or tilting degree of an optical disk.
Typically, an optical pickup apparatus which adjusts the diameter of an optical beam from a light source by using objective lenses with different numerical apertures, can access both the CD and the DVD.
Accordingly, two different kinds of objective lenses having different numerical apertures from each other can be used. However, an optical pickup apparatus that can access both the CD and the DVD by coating one objective lens with a SWP filter has been suggested.
FIG. 1 illustrates the structure typical of a known optical pickup apparatus for use in a combination CD/DVD mode.
Referring to FIG. 1, the optical pickup apparatus includes a light source 1 for emitting a light for use in a DVD, a light source 2 for emitting a light for use in a CD, a beam splitter 3 for transmitting an optical beam that has been emitted from the light source 1 and reflecting an optical beam that has been emitted from the light source 2, a collimating lens 4 for progressing an optical beam from the beam splitter 3 into a parallel beam, a half mirror 5 for converting a light path by reflecting the parallel beam that has passed through the collimating lens 4, an objective lens 6a for converging the beam reflected by the half mirror 5 reflected to an arbitrary spot on a disk 7, and a short wavelength pass (SWP) filter 6b for adjusting a numerical aperture NA of the objective lens 6a. 
An optical beam emitted from the light source 1 for use in a DVD passes through the beam splitter 3 and becomes a parallel beam due to the collimating lens 4. On the other hand, an optical being emitted from the light source 2 for use in a CD is reflected by the beam splitter 3, and is made a parallel beam by the collimating lens 4. When the parallel beam passes through the collimating lens 4, the half mirror 5 verticalizes the optical path of the parallel beam, and the optical beam (via the half mirror 5) converges to one spot on the disk 7 via the objective lens 6a. At this time, a CD/DVD common access through selective filtering of 650 nm (DVD) and 780 nm (CD) can be made by passing the incident optical beam upon the objective lens 6a by way of the half mirror 5 through the SWP filter 6b. 
When taking into consideration a compatible use of CD and DVD, consideration must be given to difference of numerical apertures (NA), difference of wavelengths, and difference of disk thicknesses (t).
This is even more so in a case of an objective lens for use in a HD for recording/reproducing, wherein a NA is 0.85, the wavelength (λ) is 405 nm, and the disk difference (t) is 0.1 mm. On the other hand, in a case of an objective lens for use in a DVD, NA is 0.6, the wavelength (λ) is 650 nm, and the disk difference (t) is 0.6 mm. Therefore, a special kind of means for overcoming the differences of NA, wavelength (λ), and t is required for the compatible use of HD and DVD. In short, the spherical aberration caused by the difference in the thicknesses of a disk and the chromatic aberration caused by the difference in laser wavelengths should be compensated in order to reproduce a disk for use in DVD.